The present invention generally relates to a method and device for cruise control governors and, more particularly to a cruise control governors using droop selection logic.
The present invention also relates to a computer program product and a storage medium for a computer all to be used with a computer for executing said method.
As is known in the art, a cruise control governor attempts to maintain a user-selected vehicle speed. Referring to FIG. 1 (a), if the vehicle speed maintained by the cruise control governor is plotted as a function of time, it is apparent that the actual vehicle speed is not perfectly maintained at the cruise control set speed, because the controller can only attempt to maintain the desired set speed by measuring deviation of the actual speed from the set speed. The governor attempts to maintain a constant vehicle speed by controlling the amount of fuel which is provided, to the engine, which is roughly proportional to the amount of torque that the engine will generate. FIG. 1 (b) plots the engine torque vs. time which corresponds to the vehicle speed plot of FIG. 1 (a). If the vehicle speed is plotted against engine torque, as in FIG. 2, a convenient paradigm is provided for visualizing the action of the cruise control governor. Viewing the cruise control governor from the perspective of FIG. 2 indicates that the engine will produce whatever engine torque is required to maintain a constant vehicle speed. Since the torque that goes into the vehicle varies with the terrain, the torque generation from the engine must also vary in order to maintain a constant vehicle speed.
Cruise control governors are devices that attempt to maintain a desired set speed condition by monitoring, the system that they are trying to control. The cruise control governor monitors the road speed of the vehicle and reacts by changing the fuel command to the engine. For example, when the governor detects an underspeed condition, the governor increases the torque generation of the engine in order to increase the speed of the vehicle, thereby compensating for the undesirable underspeed situation. Thus, the governor is not capable of reacting until it recognizes that the vehicle has already deviated from the set speed. Once the vehicle has deviated from the set speed, it is too late for the governor to provide a perfect response, therefore the governor attempts to return the vehicle to the set speed as quickly as possible. Because the vehicle, must deviate from the set speed before the governor reacts, it is impossible for the governor to provide a perfect response. This is why the plot of vehicle speed vs. time in FIG. 1 (a) exhibits slight deviations both above and below the vehicle set speed. FIG. 3 is a process flow diagram which illustrates the interaction of the governor 22 with the vehicle/engine combination 24. The actual measured vehicle speed is subtracted from the desired set speed (which is set by the driver using the cab interface 20) in order to create a speed error signal. This speed error signal is input to the governor 22, which adjusts the fuel command signal to the vehicle/engine combination 24 in response thereto.
The plot of engine torque vs. vehicle speed in FIG. 2 is referred to as a “droop” curve. Such a droop curve is realized because the controller is attempting to follow a goal droop curve. The controller adjusts its response, and thus the response of the engine, as a function of the current operating conditions of the vehicle and as a function of the goal droop curve. FIGS. 4a-f illustrate examples of various goal droop curves. The shape of the goal droop curve used with any particular controller depends upon the particular response that is desired from the controller.
The ability for the controller to follow the goal droop curves depends upon the gain of the governor. The governor's gain is an indication of the aggressiveness of the controller. A high gain provides a very aggressive governor that will adjust engine torque generation rapidly in an attempt to follow the goal droop curve. However, aggressive gain governors also have a tendency to be unstable. In summary, the goal droop curves define where the controller attempts to maintain vehicle operation, and the governor gains define how aggressively the goal droop curves are followed.
Because vehicle speed determines where on the goal droop curve the controller attempts to operate, environmental factors which affect the speed of the vehicle affect the performance of the controller. One such environmental factor is the grade of the road surface upon which the vehicle travels. Gradability is a concept that allows one to consider the relationship between vehicle speed, the grade of a hill, the full torque curve of the engine, aerodynamic drag, gearing and torque requirements. This concept utilizes a grade curve as illustrated in FIG. 5. The grade curve denotes the torque needed, at every speed, to remain at an equilibrium for a certain combination of hill grade, aerodynamic drag, and gearing selection. FIG. 6 shows some examples of how various hill grades affect the placement of the grade curve. Such grade curves are useful because they provide an easy means to determine if the vehicle is going to accelerate or decelerate. If at the current vehicle speed, the grade curve is higher than the torque curve, then the vehicle will slow down to the point of intersection between the grade curve and the torque curve. If, at the current vehicle speed, the grade curve is lower than the torque curve, then the vehicle will accelerate to a vehicle speed where the grade curve and the torque curve intersect. FIG. 7 shows an example of such movement.
When the vehicle goes over a hill, the grade varies depending upon where on the hill the vehicle is placed. FIG. 8 shows the various grades which are encountered by the vehicle on a symmetrical bill. As illustrated in FIG. 9, the grade curve for a vehicle progressing to the top of a hill will move to the left as the maximum percent grade is reached, and then move back to the right as the grade is decreased back to zero, lithe vehicle slows down at all before the crest of the hill, due to the higher torque requirements, then the vehicle will accelerate before the top of the bill because the grade curve moves to the right as the vehicle approaches the crest of the bill (0% grade). The exact location of the start of the acceleration will depend upon the shape and length of the hill, the rating of the engine, and the aerodynamics of the vehicle.
Because most hills are relatively symmetrical and follow the model of FIG. 8, acceleration of the vehicle as it nears the crest of the hill is undesirable due to the fact that the vehicle will accelerate automatically on the downside of the hill due to the negative grade. Conversely, a vehicle entering a valley will decelerate on the downside of the hill prior to its eventual automatic deceleration when it encounters the upside of the hill on the opposite side of the valley. When a vehicle accelerates prior to a point where the terrain will cause the vehicle to accelerate automatically, or when a vehicle decelerates prior to a point where the terrain will cause the vehicle to decelerate automatically, fuel is wasted.
U.S. Pat. No. 5,868,214 discloses an example of prior art where a cruise controller is able to recognize that the vehicle is cresting a hill or approaching the bottom of a valley, and thereby alter the performance of the cruise control governor in order to obtain maximum fuel economy throughout the entire hill or valley event. The cruise control governor is able to dynamically define and switch between various goal droop curves in order to find the best goal droop curve for use with the current vehicle driving situation. For instance, different goal droop curves will dynamically be defined and selected when the vehicle is lugging up a hill, coasting down a hill, cruising on level ground, preparing to crest a hill, or preparing to transition off of a downhill slope. When said cruise control governor is applied in a vehicle with a stage-geared automatic transmission and since the vehicle speed is allowed to drop (top droop) when climbing a hill and in order to reach maximum engine torque output, said temporary vehicle speed decrease often can result in a downshift. Many downshifts are necessary in order to be able to climb the hill but there are also many downshifts that are unnecessary and which result in decreased fuel efficiency. Further, when approaching an uphill and when the speed of the vehicle is bigger than set speed, torque can still be delivered from the engine to driving wheels due to the bottom droop curve. This gives a higher vehicle speed later at the crest of the hill. The higher vehicle speed at the crest sometimes results in that a downshift can be avoided. But there are also cases where no downshift would have occurred even if the vehicle speed had been lower. This means that fuel was spent unnecessarily when driving with this higher vehicle speed.
The present invention is directed, according to an aspect thereof, toward presenting as cruise control governor that is able to increase fuel efficiency further.
According to a first aspect of the invention, there is provided a method for controlling a cruise control governor operable to maintain a set speed of a vehicle by commanding fueling to an engine of the vehicle according to a plurality of goal droop curves, where said goal droop curves comprise:                an isochronous droop curve which coincides with the set speed and is bounded by a first point below a full torque curve of the engine and by a second point above a zero torque curve of the engine;        a top droop curve which is bounded by the first point and by a third point on the full torque curve;        a bottom droop curve which is bounded by the second point and by a fourth point on the zero torque curve, the method comprising the steps of:                    performing command according to said droop curves;            registering that the vehicle soon will enter an uphill slope;            estimating in a vehicle position before entering said uphill slope if a downshift in a transmission of the vehicle will occur when traveling said uphill slope during a coming time period;                        if said downshift is estimated to occur then performing a fuel saving action during said time period in order to avoid said downshift.        
In another embodiment of the invention said fuel saving action being to during said time period performing command according to a second isochronous curve instead of performing command according to at least one of or both of said top and bottom droop curves.
In a further embodiment of the invention said second isochronous curve coincides with said set speed and being extended from said second point and up to said full torque curve of the engine, when performing command instead of according to said top droop curve.
In another embodiment of the invention said second isochronous curve coincides with said set speed and being extended from said first point and down to said zero torque curve of the engine when performing command instead of according to said bottom droop curve.
In a further embodiment of the invention said second isochronous curve coincides with said set speed and being extended from said full torque curve of the engine and down to said zero torque curve of the engine when performing command instead, of according to said top and bottom droop curves.
In another embodiment of the invention said fuel saving action being an adaptation of downshifting limits of said transmission in order to avoid downshifting limits to occur at or above said top droop curve.
In a further embodiment of the invention said adaptation is lowering the downshifting limit to a position below said top droop curve during said tune period.
In another embodiment of the invention said top droop curve being a top dynamic droop curve which is dynamically defined during operation of the vehicle and is bounded by a sixth point on the full torque curve to the left of the third point and by a fifth point on the isochronous droop curve between the first and second points.
In a further embodiment of the invention said bottom droop curve being a bottom dynamic droop curve which is dynamically defined during operation of the vehicle and is bounded by a seventh point on the zero torque curve to the right of the fourth point and by an eighth point on the isochronous droop curve between the second and fifth points.
The present invention also relates to a vehicle comprising a cruise control governor and where a control unit is programmed to perform the steps of said method.